This invention relates to a method for forming light weight, high strength composite structures having complex, enclosed, internal features.
Enclosed structures made from thermo-setting resin impregnated structural fabric have long been produced by laying up external and internal plies of fabric within molds that include internal forms made from materials that melt at a temperature higher than the resin cure temperature but lower than the resin degradation temperature. U.S. Pat. No. 2,755,216 issued to Lemons teaches a process for forming a radome shell having internal, reinforcing walls. Lemons"" shell is made out of fiberglass cloth impregnated with a thermosetting resin. In Lemons""process, resin impregnated sheets of fiber glass cloth are wrapped around elongated, rectangular wax mandrels. The wrapped mandrels are then laid up between resin impregnated sheets to complete a contoured structure. The resin in the Lemons lay-up is then cured at a temperature below the melting point of the wax mandrels. As the lay-up cures the resin that saturates adjacent sheets of material fuses and bonds while it hardens. Once the resin is cured, the entire lay-up is raised to a temperature that is above the melting point of the wax mandrels so that the wax mandrels can melt and drain out of the completed radome structure. The completed radome structure produced by the Lemons process has two shell walls that are reinforced and connected by a series of connecting walls that remain after the removal of the wax mandrels.
As noted above, the prior art includes processes that use destructible core molds for forming internal voids in composite structures. However, the prior art does not teach the use of extensive, one piece destructible core molds to produce large panel structures having internal reinforcing layers. The panel making process of the present invention is a method for making a three layered panel that has top, bottom and core sheets that are all made from resin impregnated structural fiber cloth. The top sheet is formed into a substantially flat or smoothly contoured shape. The bottom sheet is spaced from the top sheet. The core sheet joins and inter-supports the top sheet and the bottom sheet. It is formed into a three-dimensional shape having alternating elevations and depressions which vary about a neutral surface that is located between the top and bottom sheets. The depressions of the core sheet have flat valley areas that are bonded to the bottom sheet while the elevations of the core sheet have flat peak areas that are bonded to the top sheet. The core sheet is also a barrier between a lower cavity and an upper cavity. The lower cavity is open at the edges of the panel and is bounded by the bottom sheet and the core sheet. Similarly, the upper cavity is also open at the edges of the panel and is bounded by the top sheet and the core sheet.
The present invention method is most particularly concerned with the method for forming the core sheet and bonding it to the top sheet and the bottom sheet. The top, bottom and core sheets are all formed from fiber reinforced composite materials that include structural fiber fabric that is impregnated with thermosetting resin. This liquid thermosetting resin is cured to into a strong, rigid material when it is heated to a curing temperature. The method for making the panel of the present invention requires the use of an upper mold or a bag press for forming the top sheet, a lower mold for forming the bottom sheet, a first core mold and a second cold mold. The first and second core molds are opposite and symmetrical. They are substantially solid, having a flat surface on one side and a contoured surface with regularly alternating elevations and depressions on the other side. The elevations of each core mold include generally flat peak areas. The depressions of each core mold have openings that break through the flat surface of the core mold. The core molds fit together in an opposing fashion. The first core mold can also be understood as a solid that corresponds to the lower enclosed space between the bottom sheet and the core sheet of the finished panel. In the same way, the second mold can be understood as a solid that corresponds to the upper enclosed space between the core sheet and the top sheet of the panel. Because the volume between the core sheet and an adjacent top or bottom sheet of the panel is continuous and therefore not divided into more than one compartment, it is possible to use just two core molds to produce a panel having a large area. It is therefore not necessary to have a large number of separate core molds to lay up the sheets of the panel.
The core molds must be made from a material that is impermeable to the thermosetting resin that is used to impregnate the top, bottom and core sheets. The core molds must also be made from a material that has a melting point that is above the temperature at which the thermosetting resin cures and that is also below the temperature at which the cured resin begins to degrade. The presently preferred material for fashioning a core mold is a wax that melts at a temperature that is above the resin curing temperature and below the temperature at which the cured resin begins to degrade. Many of the resins known to those skilled in the art cure rapidly at temperatures below 160xc2x0 F. and, once cured, will not degrade at temperatures that are twenty or thirty degrees above 160xc2x0 F. Accordingly, good results can be obtained by making a core mold from wax that melts well above 160xc2x0 F. Those skilled in the art routinely formulate waxes that will melt above selected temperatures. For example, a wax formulation consisting of 40 parts Calwax 126(trademark) wax, 60 Parts Calwax 252B(trademark) wax and 1 part Calwax 320(trademark) wax will melt above 160xc2x0 F. However, it should be noted that this formulation is only an example and that a myriad of waxes could be formulated having acceptable or even more desirable characteristics for the purpose of this invention.
The panel is laid up by first placing a first dry sheet of structural fiber cloth on the lower mold surface. This first dry sheet will become the bottom sheet of the finished panel. The first core mold is placed on top of the first dry sheet of structural fiber cloth so that its peaks are oriented away from the lower mold surface. A second sheet of dry structural cloth is place on top of the first core mold. This second dry sheet will become the core sheet of the finished panel. Then, a second core mold is placed on top of the second sheet in an opposite, fitting relationship with the first core mold. Because the first core mold has openings at its valleys, the peak areas of the second core mold push the second sheet of cloth into contact with the first sheet at each of the valleys of the first core mold. A third sheet of structural cloth is then placed on top of the second core mold. This third sheet will become the top sheet of the finished panel. The second sheet is in contact with the third sheet where the valleys in the second core mold have openings and where the peak areas of the first core mold push the second sheet into contact with the third sheet. Either a bag press or a second mold is placed on top of the third sheet. Pre-impregnated sheets of structural cloth can be used for the top and bottom sheets and possibly even for the core sheet. However, a pre-impregnated sheet of cloth may not have the flexibility needed to conform to the bumpy contours of the core sheet.
It may also be necessary to pre-form a core sheet in a progressive molding or stamping process so that a core sheet conforms to the peaks and valleys of the core molds. Continuous fiber material does not easily assume a core shape having a deep draw over a large surface area. Accordingly, in a large lay-up having a relatively large core thickness it may be necessary to pre-form a core sheet using a progressive, stamping process or even a progressive, heated stamping process. A heated process is particularly useful when forming a continuous woven fiber core sheet that includes a binder such as a thermoplastic or a poly-vinylalcohol. A core sheet fashioned from a mat of short, non-continuous fibers would be relatively easy to form into a core sheet in the above described steps.
After the lay-up is completed, liquid resin is transferred into the dry structural cloth through holes or channels in the upper or lower molds. The resin is such that it can be cured at a temperature below the melting point of the core mold material. After the resin is cured, the panel is removed from the lay-up and heated until the core mold material melts and drains out. As stated above, the temperature at which the core mold material melts is a temperature that does not harm the cured resin. A solvent may be used to wash out the remaining core mold material. The melted core mold material may be recovered and recycled for later use.
The resulting formed panel, if made using appropriate geometry and combinations of structural fiber cloth and resin systems, can be extremely strong and light. A panel structure made using the process of the present invention could even be produced in very large quantities in a continuous process resembling an extrusion process where elements are rolled together while being saturated with resin, cured and then drained of core mold material to produce a panel product.
In a second embodiment of the present invention, large hollow structures can be produced having enclosed internal structures. The method of the second embodiment includes the formulation of a core mold wax which melts above 160xc2x0 F. and which contains between 10 and 60 parts ceramic micro-spheres. As noted above, a wax formulation that melts above 160xc2x0 F., may consist of 40 parts Calwax 126(trademark) wax, 60 Parts Calwax 252B(trademark) wax and 1 part Calwax 320(trademark) wax. The ceramic micro-spheres stabilize the thermal expansion characteristics of the destructible core mold wax composition so that large, precise structures can be molded with complex internal shapes. The panel described above as well as other larger structural elements can be molded using such a micro-sphere enhanced core mold composition.
The method of the present invention can be used to produce large, lightweight, high strength panels at a relatively low cost. Because the core molds can be made in single, large pieces, the time and labor involved in laying up a panel is greatly reduced. Structures made using the method of the present invention can be adapted for a wide range of applications. Only a few of the possible applications would include acoustic panels, structural panels and sound and impact energy absorbing panels.